1. Field of the Invention
The invention relates to a charging method, and more particularly to a charging method and system for charging a battery module.
2. Brief Description of the Related Art
Conventional methods for charging batteries include limited-current charging, pulse charging, and multistage constant-current charging. FIGS. 1a and 1b show the status of batteries as they are charged utilizing the limited-current charging method.
As shown in FIG. 1a, since the batteries are not balanced, output voltages 1 to 3 of different batteries are different. A power supply provides a charge current 4 to the batteries according to the limited-current charging method. Before time t1, the power supply provides the constant charge current 4 to the batteries. When the output voltage arrives at a preset voltage, the power supply stops providing the charge current. In the limited-current charging method, only one battery is fully charged while the others are not.
As shown in FIG. 1b, all the batteries are balanced such that output voltages therefrom are almost equal. Thus, a curve 10 represents an output voltage. Before time t2, the power supply provides constant charge current 11 to the batteries. After time t2, when the output voltage 10 arrives at a preset voltage, the charge current 11 is reduced according to a voltage difference between a power supply and the batteries. When the voltage difference is less, the charge current is lower. Although the output voltage 10 arrives at the preset voltage, the power supply still provides the lower charge current to the batteries. Thus, all the batteries are fully charged but the charge time is increased.
When the batteries are continuously idle for thirty minutes, a relaxed battery voltage will be as the curve 12 shown in FIG. 1b. The power supply provides the charge current according to the open-circuit voltage.
FIG. 2 shows the status of batteries as they are charged utilizing the pulse charging method. All the batteries are balanced such that output voltages from the batteries are the same shown as curve 20. A power supply provides a pulse charge current 21 to the batteries. Relaxed battery voltage 22 is undetectable when the batteries are charged by the pulse charging method. Therefore the relaxed battery voltage 22 may easily exceed a preset voltage as shown at time t3 in FIG. 2, causing danger. At timing t3, the power supply stops charging the batteries, and at time t4, an output voltage is detected. An interval about 30 minutes between time t3 and t4 is generally required. In other words, the pulse charging method cannot immediately detect output voltages.
FIG. 3 shows the status of batteries as it is charged utilizing the multistage constant-current charging method. A power supply provides a varied charge current 31 for the batteries according to output voltages 30. Curve 32 shows a relaxed battery voltage. Since this method provides varied constant charge currents, the charge control circuit is complex and costly.
Conventional charging methods have different drawbacks. In the limited-current charging method, when batteries are not balanced, only one battery is fully charged and when batteries are balanced, the charge time is longer. In the pulse charging method, output voltage is not immediately detected and charge time is difficult to control. In the multistage constant-current charging method, the charge control circuit is complex and costly.